Evaluation of in situ chemical fixation for remediation of arsenic-contaminated soil

Soils from many industrial sites are contaminated with arsenic because of the application of herbicide containing arsenic trioxide. Soil samples from two representative sites, FW and BH, were collected to characterize the occurrence of As in the contaminated soils and to develop a cost effective treatment method for soil remediation. The soils are both sandy loams with environmentally available arsenic levels of up to 325 mg/kg (FW) and 900 mg/kg (BH). Analysis of the soils by scanning electron microscope coupled with energy dispersive X-ray spectroscopy and by a 4-step sequential chemical extraction procedure indicated that most of the soil arsenic is disseminated on the surfaces of fine-grained soil particles, with a significant portion in close association with amorphous Al and Fe hydroxides. Synthetic acid rain (EPA Method 1312 SPLP leachate) sequential leaching batch experiments showed that only a small portion of the total extractable soil arsenic was potentially mobile. A low cost in situ chemical fixation (ISCF) treatment method was designed to react contaminated soils directly with chemical fixation solutions to form insoluble arsenic-bearing phases and thus decrease the environmental leachability of As. Three sets of fixation solutions were used in batch experiments to evaluate the optimal combination of reagents: (1) FeSO4; (2) FeSO4 and KMnO4; (3) FeSO4 KMnO4 and CaCO3. Among all combinations of fixation solutions, the treatment with only FeSO4 had the best effect, reducing SPLP-leachable arsenic as much as 90%. However, no evidence was found for the formation of any crystalline As and Fe bearing phases during the treatment process. Sequential chemical extractions of the treated soils indicated that a significant portion of labile soil arsenic was transferred to be associated with the hydrous Fe oxides (HFO), produced by ISCF treatment. Synchrotron radiation X-ray absorption spectroscopy analysis further revealed that soil arsenic is present primary as arsenate and 'fixed' by forming inner-sphere bidentate surface complexes with the produced HFO. Analysis of the aged treated soil samples showed no significant variation in the arsenic chemical bonding environment compared to newly treated samples, suggesting that the arsenate surface-complexes on HFO should show long-term stability in the vadose zone.